NARAYANGURI, India, May 5 (Reuters) - Families of Muslim villagers massacred on the fringes of an Indian national park were burying their loved ones after a spasm of ethnic violence in the northeastern state of Assam cast a shadow over the world's biggest ever election.

On Monday, villagers in Narayanguri laid to rest a baby girl, one of at least 21 victims of a massacre by suspected tribal militants who threw dead and dying into blazing huts and gunned down women and children as they fled towards a nearby river. Many of the other dead were buried on Sunday.

"I've seen militant attacks before, but never such a massacre," said local police chief Naren Bora, who led a small group that was the first to arrive at the scene on Friday.

At least 13 people were killed in attacks on Muslims in two other villages in the area, taking the death toll from last week's violence to 34.

The corpses of two women were found on Sunday floating in the Beki river that flows through Narayanguri. A man's body was found elsewhere in the same district, police said.

India is in the home stretch of a five-week general election, which has heightened ethnic and religious tensions in some parts of the country and in which the opposition Bharatiya Janata Party (BJP) looks set to emerge as the biggest group.

The Hindu nationalist BJP has condemned the killings and accused the Congress party, which runs Assam and leads the national government, of doing too little to contain an incipient insurgency in the state.

The BJP's candidate for prime minister, Narendra Modi, has also ramped up verbal attacks on illegal immigration by Muslims from nearby Bangladesh, drawing criticism from his opponents for inflaming tensions.

Speaking in the next-door state of West Bengal on Sunday, Modi accused the state government of being soft on "infiltrators" from Bangladesh. Modi last week said illegal immigrants living in West Bengal should have their "bags packed" in case he came to power. He made no reference to the massacres.

Critics have long accused the BJP of deep-seated prejudice against Muslims, who make up more than 12 percent of India's 1.2 billion people. The BJP says it only opposes preferential treatment for any community.

FANNING OUT

Wrapped in black plastic sheeting, 18 victims were lowered into the ground on Sunday in Narayanguri, a popular picnic spot close to one of the entrances to the Manas national park. Known for dense biodiversity, tigers, and elephants, the UNESCO world heritage site is visited by Indian and foreign tourists.

"I was cycling home when I suddenly heard shots and saw the flames," said Shalam Gaon, the 40-year-old village head. "We'll never go back to live there now."

Security forces, including soldiers and elite border police, fanned out across Manas national park on Monday, setting up camps and hunting for rebels suspected of the attack, state and local police said.

"Operations are on in all sensitive areas," said Rajen Singh, commander of Assam's 14th police battalion. "These are offensive operations."

Security forces temporarily lifted a curfew but were due to reimpose it before nightfall. Muslims protesting the killings scuffled with police after blocking the roads in several districts of Assam on Monday morning.

Sporadic outbreaks of violence are common against Muslims who live alongside the Bodo tribe in western Assam, near the borders of Bangladesh and the Himalayan foothills of Bhutan.

Bodo militants say the Muslims are illegal immigrants from Bangladesh who have taken ancestral lands, while members of the minority group say they were mostly born in India.

FOREST GUARD CLAIM

Several villagers said the attackers were forest guards who lived in a camp in the village and were well known to them.

"They wore black cloths across their faces, but I knew who they were, I used to sit and chat with them every day," said Safiqul Islam, whose sister and her son were killed.

Assam's Additional Director General of Police, A. P. Rout, said six forest guards were being interrogated about the villagers' claims.

"These are very serious accusations. They are government employees, they have authorized weapons. It could be wild allegations, but we will find out," Rout told Reuters.

More than 20 people have been arrested for abetting the attackers, and more arrests are expected, Rout said.

Three militants from the National Democratic Front of Bodoland (NDFB) died in gun battles with the police in a different part of Assam on Sunday, Rout said. He said the fighting did not seem to be related to the massacres.

The NDFB has denied responsibility for attacks on Muslims. (Editing by Douglas Busvine and Raju Gopalakrishnan)